. Reading the number letters on the casting.... looking at casting numbers The first letter is the month the second two numbers is the day and the last single number is the year.. then there is a casting that looks like a dotted circle with a pointer In the center this indicates day or night shift time....you just have to worry about the right date....examples....H227 h is October 22 is the 22nd day and 7 is 1967. ....A189. Is January 18 1969.. from what I understand it’s about 3 to 4 weeks before the build date of the car is a good guideline ..getting correct carbs aren’t cheap. I waited for 15 years to get the correct HO carb for mine.. it’s worth it to me...